You are correct, it should be either "Second World War" or "WW2/World War 2". Both "second" and "two" is wrong - we have only had one Second World War! (Thank God!)
I don't think it is likely that someone was joking when they said that though, unless the context was very ususual. It's more likely that they started to say Second World War, then changed their mind half way through, and managed to confuse themselves and ended up saying both by accident.
Interested in Language